The invention is based on a corona ignition system for the ignition of fuel in a combustion chamber of a cyclically operating internal combustion engine, as is known from WO 2010/011838 A1.
WO 2010/011838 A1 discloses a corona ignition system with which a fuel-air mixture in a combustion chamber of an internal combustion engine can be ignited by a corona discharge generated in the combustion chamber. This corona ignition device has an ignition electrode that is held in an insulator. The ignition electrode forms, together with the insulator and a sheath enclosing the insulator, an electrical capacitance. This capacitance is part of an electrical oscillating circuit of the corona ignition device, which undergoes excitation with a high-frequency alternating voltage, from, for example, 30 kHz to 5 MHz. This leads to a voltage excess at the ignition electrode causing a corona discharge.
Thus, a high-frequency corona discharge can be generated in the combustion chamber. The corona discharge should not turn into an arc discharge or a spark discharge. Therefore, it is ensured that the voltage between the ignition electrode and the ground remains below the breakdown voltage.
WO 2010/011838 A1 discloses that the frequency of the oscillating circuit is regulated measuring the phase shift between current and voltage at the feeder points of the oscillating circuit and regulating the phase shift to zero by means of a phase control loop, since, in a series oscillating circuit, power and voltage are in phase in resonance (phase shift=zero). The phase control loop controls the switching frequency of a switching device, with which a predetermined voltage is applied alternatingly to one primary winding and to the other primary winding of the transformer, such that current and voltage are in phase with each other on the secondary side of the transformer at the feeder points of the series oscillating circuit.
In prior art, the shift of the resonant frequency of the high-frequency oscillating circuit, which contains the high-frequency igniter, is a significant problem. There are various causes of this. One cause for the shift of the resonant frequency is changes in the combustion chamber of the internal combustion engine, for example changes of the temperature, the pressure, the moisture level, the tip or tips of the ignition electrode of the high-frequency igniter becoming dirty, and changes of further parameters that are dependent on the operation of the combustion engine. Also, the fact of corona formation may shift the resonant frequency. Updating the excitation frequency to the resonant frequency by a phase control loop, as is disclosed in WO 2010/011838 A1, is expensive, and only partially solves the problem. The phase control is susceptible to a temperature drift of the components of the phase control loop and to voltage noise.
In order to avoid the disadvantages of a phase control loop, it is known from DE 10 2011 052 096 A1 to monitor the instantaneous values of current or voltage of the oscillating circuit and to excite the high-frequency generator with primary voltage pulses, which are each begun or terminated when the instantaneous value of power or voltage exceeds or falls below a predetermined switching threshold. This method has the disadvantage of requiring sophisticated measuring technology.